Application of a larval medaka assay to evaluate the fish safety level in Sagami River, Japan.

Despite of the increasing concern on aquatic life preservation from water pollution, few methods are available to evaluate the fish safety level of river water. In this study, a principal urban river of Japan, Sagami River, was investigated the geographical and seasonal changes in the fish safety level using an efficient larval medaka (Oryzias latipes) assay. Then combined with topography and land-use character of the catchment, the influence of human activity was discussed. The results revealed the highest fish safety levels and the lowest dissolved organic carbon (DOC) concentrations in the preserved region of the upstream. In the lower mainstream and the west tributaries, catchments of which were developed regions with high sewerage diffusion rates, the fish safety levels slightly decreased although the DOC concentrations were low. By comparison, the east tributaries, which were allocated in highly developed regions without sufficient sewerage service, showed not only high DOC concentrations but also greatly decreased fish safety levels. Two small east tributaries showed three to four times increase in the toxicity in winter, as influenced by the irrigation activity on the bank. The safety level did not closely depend on the common water quality indicator of DOC, although extremely high DOC concentrations tended to relate to low safety levels. The data is useful for better management of the river studied, and the approaches used in this paper will give a hint to the management of other rivers.